By St Peter of Damaskos - Philokalia Book1
True Discrimination characterized by an unerring recognition of what is good and what is not, and the knowledge of the will of God in all that one does. Spiritual insight is characterized, first, by awareness of one’s own failings before they issue in outward actions, as well as of the stealthy tricks of the demons; and, second, by the knowledge of the mysteries hidden in the divine Scriptures and in sensible creation.
As has been already explained, humility, the mother of discrimination and spiritual insight, likewise has its own characteristic by which it is known. The humble person; must possess every virtue and yet truly think himself the greatest of debtors and inferior to everything else in creation. If, however, a person does not think in this way, then he can be assured that he is mi fact inferior to everything else in creation, even though he seems to lead a life like that of the angels. For even a true angel possessing so many virtues and so much wisdom cannot conform to the Creator’s will unless he also possesses humility. What, then, can a person who thinks that he is an angel say for himself if he lacks humility, source of all present and future blessings, begetter of that discrimination which illumines the ends of the earth and without which all things are obscure?
Discrimination is not only called light; it truly is light. We need this light before we say or do anything. When it is present we are able to view everything else with wonder. We can marvel at how God, on the first and greatest of days, began by creating light, so that what was subsequently created might not he invisible and as if it did not exist, as St John of Damaskos says.’ Let it be said again: discrimination is light; and the spiritual insight it generates is more necessary than all other gifts. For what is more necessary than to perceive the wiles of the demons and with the help of God’s grace to protect one’s soul? Other things most necessary to; us include, according to St Isaac, purity of conscience; and, according to the apostle, the sanctification of the body (cf. Rom. 12:1; 1 Cor. 6:19-20) without which ‘no one will see the Lord’ (Heb. 12:14).